How can I tell if a Caster Wheel is made of rubber or polyurethane?

I have a lot of wheels that I want to sell, but do not know if the wheels of rubber or polyurethane. Is there any evidence or touch the main visual difference / between the two materials that will help me determine what are those facts?

Rubber strain crystallizes. One of you put wheels under intense pressure, for example a press, then, if it crystallizes, which means they shatter, then you have rubber. Both materials are polymers and this is the only way I can think of giving the information I have. Hopefully, a expert will come and handle this.

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